This invention relates to an imaging system, and more particularly, to a method used in a video surveillance system to prevent false alarms caused by the scene under observation changing due to flickering lights, as opposed to changes in the scene resulting from the presence of an intruder.
In co-pending and co-assigned U.S. patent application Ser. No. 08/772,595, there is described a video imaging system and method of image processing which is particularly useful in video surveillance security systems. As described in the co-pending application, changes between one frame of observed video and another are detected and isolated. If the changes result from a fluctuation in the irradiation of a surface within the field of view of the sensor obtaining the image, as opposed to the introduction of a new reflecting surface (possibly an intruder) into the field of view, the isolated change is recognized as such, and no alarm is triggered by the system. This has the desirable effect of reducing the number of false alarms to which a system monitor or operator must respond.
The methodology employed by the surveillance system to process the received video images is based upon the "retinex" theory developed by Edwin Land. According to the theory, a human observer of a scene can detect differences in reflectance between two surfaces, even if the amount of visible-light energy reflected from the respective surfaces is the same. Application of the retinex theory to areas of detected differences within an observed scene is described in the co-pending application.
One situation which must be addressed by any video surveillance security system, in preventing false alarms, is that of an intruder entering an observed scene which is continually subjected to transient lighting changes. An example of such a situation is where there is a flickering light (an advertisement sign, for example) which constantly switches on and off, resulting in fluctuating lighting conditions. The improvement of the present invention allows the security system to react to an environment subjected to such continuously changing lighting conditions, and to detect an intruder if one is present; but otherwise to not produce false alarms.